This invention relates to an improved process for producing a glossy, tough, optionally smooth surface on a lignocellulosic panel such as wood, plywood, fiberboard or particleboard.
Untreated surfaces of conventional plywood, fiberboard, or particleboard, as indeed those of solid wood itself, are known to have low resistance to abrasion and to be prone to dimensional change on contact with moisture. Such dimensional changes involve local areas of relative swelling and shrinking resulting in visual deficiencies which are known as face checking, grain raising and the like. Thus, the surfaces of particleboards and fiberboards may sometimes have low decorative value even though they are not made unsightly by too many knots or knotholes.
Various methods have been used conventionally to provide coatings for panels with the purpose of overcoming some of the aforementioned surface deficiencies. Various types of paints, varnishes, lacquers and the like have been used. In particular among these, amino modified alkyd resin compositions with solvent and optionally with pigment have been applied by spraying or roller coating and then oven treating to evaporate the solvent and cure the polymer to provide a baked enamel finish. Such finishes are sometimes poor in wearability and in resistance to solvents and chemical stains. The evaporation of organic solvents during application in drying causes an air pollution problem. Furthermore, unless the panel surface is first smoothed by a sanding procedure, the surface roughness is telegraphed to the surface of the baked enamel finish and the enamel surface has an exagerated roughness which is a replica of the roughness of the original panel.
When conventional oil paints or varnishes are used, the improvement in appearance is only temporary. Even with up to five successive coats, the barrier to moisture may not be sufficient to prevent eventual distortion due to swelling and shrinking of the panel.
A type of surfacing known as overlay is accomplished by first treating a fibrous web such as paper or cotton fabric with a synthetic polymer of either the thermosetting or thermoplastic type and then laminating the treated web to the substrate panel. Alternatively, thermoplastic films have been used as overlays. When paper treated with melamine formaldehyde and/or phenol formaldehyde thermosetting resin is used as a overlay, improved wearability and resistance to stains and solvents are obtained. However, for best results in laminating thermosetting resins, high pressures of up to as high as 1200 psi are required. This high pressure has a side effect of tending to break down the internal bonds of the panel substrate. Furthermore, in order to obtain a uniform surface, the platen should be cooled to about 50.degree. C. under pressure after each heating cycle.
A fibrous type overlay was disclosed by Brant [Forest Products Journal 18 (5): 51-58 (1968)] using mechanical wood fibers that were precoated with a thermosetting resin. The resin bound fibers were applied as a mat which had a short duration of precuring stability. The direct application of dried phenolic resin polymer on the board surface together with wood fibers followed by hot pressing has also been tried. The aforesaid high curing pressure and repeated cooling cycles were still necessary using this approach.
Vinyl-modified lignocellulosic paper as a top layer for cylinder board is disclosed in U.S. Pats. Nos. 3,194,727 and 3,395,070.
Corresponding use of thermoplastics in resin paper overlays also has had several disadvantages. The requirement of high alpha-cellulose pulp or paper makes such a procedure expensive. The tackiness of polyester treated papersheet requires the extra complication of using cellophane separaters in handling. Paper overlays made with acrylic latices have had deficiencies in heat stability and resistance to organic solvents, even when the ratio of acrylic polymer to cellulose was limited to not more than 30%. In order to obtain a smooth, glossy surface, the platen must not be removed, after hot pressing, until it has been cooled to a temperature below the glass transition temperature of the impregnated thermoplastic polymer.
No known practicable method had been proposed whereby a particulate composition comprising cellulosic or wood particles merely admixed with thermoplastic polymer particles could be used successfully to prepare a smooth, glossy, tough surface on a wood base panel and in particular without the necessity of repeated cyclic cooling of the platen to a temperature below the glass transition temperature of the thermoplastic polymer until a co-pending application having Ser. No. 503,640 was filed on Sept. 6, 1974, now abandoned.
That application discloses a method of applying on the surface of a lignocellulosic panel a conventional resin adhesive, spreading thereon vinyl modified lignocellulosic particles made by polymerization of the ethylenically unsaturated monomers on substrate lignocellulosic particles, and subjecting the assembly to heat and pressure to form a coated panel. The preferred method of hot pressing used a platen or caul plate. A good method for preparing vinyl-modified lignocellulosic particles is a development of the process disclosed in U.S. Pat. No. 3,083,118, whereby reaction sites in and on said particles are created by deposition of reductant ions like ferrous ion in or on the particles. The reductant treated particles are slurried in the water which is substantially free of reductant ions and the subsequent addition of vinyl monomer free radical donating initiator results in a vinyl polymerization limited substantially to in situ polymerization on the particles containing the reductant ions. The thus treated particles are then separated from the aqueous phase and dried.
Several methods, mainly chemical and high energy radiation methods, are known for preparation of vinyl polymer and cellulosic grafted copolymer. These methods have been extensively reviewed by Ward, Kyle, Jr. (graft polymerization onto cellulose in Chemical modification of paper making fibers, pages 167 to 215, Marcelle, Dekker, Inc., New York). These methods are usually very sensitive to lignin and phenolic extractives of lignocellulose and the oxygen present in the reaction system. The vinyl polymer and lignocellulose grafted copolymers are usually more resistant against organic solvents and heat than the composites prepared by previously described methods; but graft copolymers are rather poor in molding properties and some of the physical properties of the molded products are not acceptable. For example, the hot pressed plastic sheet of methyl methacrylate grafted, ground woodpulp is very brittle and very poor in impact resistence.